In certain applications, such as material processing, it is desirable to have a laser whose output is restricted to the TEM.sub.00 mode. The desirability results from, among other things, the substantial absence of phase reversal across the output beam, the high spatial coherence, and the small beam divergence. However, the particular pumping geometry of the laser creates problems in utilizing and providing a TEM.sub.00 operational mode.
In the past, the pumping geometry has been generally either longitudinal or transverse. In the former geometry, a pump beam is colinear with the laser mode. With that geometry, a good transverse mode output is obtained, as well as efficient operation because all of the pump light can be coupled into the laser mode volume. The disadvantage of the longitudinal geometry is that it is difficult to scale to higher powers.
On the other hand, transverse pumping more readily allows scaling to higher powers by pumping the laser with many light sources such as laser diodes in a direction transverse to the longitudinal axis of the laser. But it then becomes difficult to achieve good efficiency and single transverse mode operation at the same time, because most of the pump power is absorbed at the surface of a gain medium in the laser. Therefore, to extract the power absorbed at the surface of the gain medium, the laser mode must have significant intensity at the surface of the gain medium. This makes TEM.sub.00 mode operation difficult because of the large discontinuity in the index of refraction at the surface of the gain medium. This can be less of a problem for higher order modes whose maximum intensity is spatially different from that of the TEM.sub.00 mode. If TEM.sub.00 mode operation is nevertheless desired, the mode must be kept from the surface by the gain medium to avoid the index of refraction discontinuity and resultant loss due to clipping of the spatial mode which might otherwise occur if the mode is on the surface. However, that results in much of the absorbed pump energy at the surface not being extracted.
One past method which attempts to solve the problem of extraction at the surface of the gain medium uses a laser mode which zig-zags down a slab by total internal reflection at the surfaces. This then allows the pump energy at the surface to be extracted. The general problems noted above with transverse pumping have also been addressed in the past by utilizing a composite rod wherein a core of the rod is absorptive and a surrounding sheath that is nonabsorptive. Further, the indices of refraction of both the core and the sheath are the same. The structure is for the purpose of reducing threshold pumping power and increasing the available output. The method, however, does not appear to address the specific problems associated with single transverse mode operation, including TEM.sub.00 mode operation, nor the extraction of energy at the surface of the gain medium.
A need still exists in the art to provide an improved method and apparatus of transverse pumping a solid state laser or other resonant device in a single transverse mode of operation, and particularly the TEM.sub.00 mode of operation, which maximizes the absorption of pump power and maximizes the output of the laser.